In the first place, a conventional transfer drum will be explained. An image forming apparatus which develops an electrostatic latent image formed on a photosensitive drum with toner, and then transfers a resulting toner image onto a transfer material wound around a transfer drum is known. In this type of image forming apparatus, as shown in FIG. 15, for example, a corona charger 102 for attracting a transfer material P and another corona charger 104 for transferring a toner image formed on the surface of a photosensitive drum 103 onto the transfer material P are separately provided in a cylinder 101 having a dielectric layer 101a, so that the attraction of the transfer material P and the transfer are carried out separately by these chargers 102 and 104.
However, in the image forming apparatus shown in FIG. 15, since the cylinder 101 serving as a transfer roller is of a single-layer structure having only the dielectric layer 101a, two corona chargers 102 and 104 must be provided inside the cylinder 101.
On the other hand, another type of image forming apparatus as shown in FIG. 16 is also known, which is furnished with a cylinder 201 of a double-layer structure having an outer semiconductor layer 201a and an inner base material 201b, and a grip mechanism 202 for holding the transported transfer material P along the surface of the cylinder 201. In this type of image forming apparatus, the end portion of the transported transfer material P is sandwiched by the grip mechanism 202 so as to be held along the surface of the cylinder 201, and the surface of the cylinder 201 is charged either by applying a voltage to the outer semiconductor layer 201a of the cylinder 201 or by triggering a discharge by means of a charger provided inside the cylinder 201, whereupon a toner image on the photosensitive drum 103 is transferred onto the transfer material P.
In the image forming apparatus shown in FIG. 16, since the cylinder 201 serving as a transfer roller is of the double-layer structure to charge the cylinder 201 when a toner image is transferred onto the transfer material P, the number of the chargers can be reduced.
Further, a conventional example will be explained with reference to Japanese Laid-open Patent Application No. 173435/1993 (Tokukaihei No. 5-173435, U.S. Pat. No. 5,390,012). This prior art proposes a transfer device for forming a color image on a transfer material. More specifically, the transfer device transfers a toner image of each color formed sequentially on an image carrier successively onto a transfer material carried by a transfer material carrier, composed of at least a foam body covered with a dielectric layer, so as to superimpose one toner image on another. In this prior art, electrostatic attraction by charge conferring means (attraction roller) is adopted as a carrying method of the transfer material over the transfer material carrier, and it is characterized by providing a space layer of at least 10 .mu.m-thick between the dielectric layer and the foam body layer to improve the attraction ability. However, the larger the interval of the space, the higher the applied voltage to electrostatically attract the transfer material onto the dielectric body, which raises a safety problem and makes the transfer device disadvantageous in terms of costs.
To be more specific, when the interval of the space between the foam body and dielectric layer is roughly set to at least 10 .mu.m as is in the above prior art, the interval of the space can be as small as some millimeters or as large as some meters. If the interval of the space is too large, the applied voltage for electrostatically attracting the transfer material to the transfer material carrier in a stable manner or a toner transferring voltage both applied during the toner transfer become so high that there arises a safety problem. Further, at least two power sources are necessary to carry out the electrostatic attraction of the transfer material to the transfer material carrier and the toner transfer stably in a satisfactory manner. Thus, the device may be undesirably upsized or become expensive.
In addition, when the foam body is used as the semiconductor layer provided between the dielectric layer and conductor layer, there occurs a transfer defect at a foam portion of the semiconductor layer on the side touching the dielectric layer, and the toner density at that particular portion drops. Consequently, unwanted spots appear on the resulting toner image transferred onto the transfer material, thereby deteriorating the image quality. This phenomenon is particularly noticeable on a half-tone image.
Further, under high temperature and high humidity circumstances, water drops are collected in the space between the dielectric layer and semiconductor layer, whereas under low temperature and low humidity circumstances, the interval of the space is reduced. Nevertheless, the above prior art is silent about the counter-measures to the image quality deterioration caused by the change in circumstances and to the change in circumstances itself, and therefore, a satisfactory image quality may not be maintained when the state of the space changes in response to the change in circumstances.